e or to
snap the driving belt, in which latter case the dynamo would come to a
stop. If by any chance the little water wheel was given a chance to
maintain itself against the blow for an instant, the dynamo, rated at
50 amperes, would do its best to deliver the 70,515 amperes you called
for--and the result would be a puff of smoke, and a ruined dynamo.
This is called a "short circuit"--one of the first "don'ts" in
handling electricity.

As a matter of fact every dynamo is protected against such a calamity
by means of safety devices, which will be described in a later
chapter--because no matter how careful a person may be, a partial
short circuit is apt to occur. Happily, guarding against its
disastrous effects is one of the simplest problems in connection with
the electric plant.

_Direct Current and Alternating Current_

When one has mastered the simple Ohm's Law of the electric circuit,
the next step is to determine what type of electrical generator is
best suited to the requirements of a farm plant.

In the first place, electric current is divided into two classes of
interest here--_alternating_, and _direct_.

We have seen that when a wire is moved through the field of a magnet,
there is induced in it two pulsations--first in one direction, then in
another. This is an _alternating_ current, so called because it
changes its direction. If, with our armature containing hundreds of
wires to "cut" the lines of force of a group of magnets, we connected
the beginning of each wire with one copper ring, and the end of each
wire with another copper ring, we would have what is called an
_alternating-current_ dynamo. Simply by pressing a strap of flexible
copper against each revolving copper ring, we would gather the sum of
the current of these conductors. Its course would be represented by
the curved line in the diagram, one loop on each side of the middle
line (which represents time) would be a _cycle_. The number of
_cycles_ to the second depends on the speed of the armature; in
ordinary practice it is usually twenty-five or sixty. Alternating
current has many advantages, which however, do not concern us here.
Except under very rare conditions, a farmer installing his own plant
should not use this type of machine.

[Illustration: Diagram of alternating and direct current]

If, however, instead of gathering all the current with brushes bearing
on two copper rings, we collected all the current traveling in one
dire